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Creating Curved Edges with Acrylic

Even though Tiny Bot is an obstacle avoiding robot, collisions with chairs, table legs, or dark/soft objects are inevitable.

Therefore, when designing the base, I decided to create curved edges on the front and back. This added a new dimension of difficulty to cutting the acrylic (plexiglass) for the base. Table saws, Chop Saws, and Radial Arm Saws all make perfectly neat straight edges but creating curved edges in acrylic needed something a little more complex….Machining.

Actually, I found machining acrylic to be quite easy. In fact, most of the work is involved in creating a template. Once this is done, you will have the template available for other robot projects.

Plexiglass Robot Bases are easy once you make the template.

Creating a Template

The first step is to create a tempate out of something easily cut and sanded to the desired shape such as 1/4″ MDF board. This template will be the base that the bearing from the router bit will travel against. This forces the blades of the router bit to make precise cuts along the shape.

After searchng through my shop, I settled on a table saw throat plate as the shape for Tiny Bot’s base. It had just the right width and roundness for my PCB and motors. I’ve also used items such as paint can lids or mayonnaise jars to trace out my shapes.

Smooth, steady pressure in a gentle arc when using a band saw.

Cutting out the Template on the Band Saw

After I traced out my shape, I cut it on a table saw to reduce the size. This made it easier to maneuver under the band-saw. The idea here is to cut as close to the pencil marks without actually cutting into them. I’m just removing as much material as possible to reduce sanding.

With a band saw, Once the blade starts in a certain direction, it’s very hard to get it to change it’s course. I’ve learned that it’s best to take your time here and apply very slight and consistent pressure on the piece to coax it around the shape. To keep the “jaggedness” to a minimum, I try to turn the piece in one sweeping motion.

A belt sander helps to remove the inevitable jagged edges from the template.

Sanding the Template

The next step is to finish the template by sanding off the blade marks or high points left on the piece. The belt sander makes easy work of this. Especially an oscillating sander. The trick here is to keep the piece moving around the arc at all times or you’ll end up with some flat spots very quickly.

Attaching the Template to the Acrylic

Once the template is complete, the next step is to attach it to the acrylic piece. Any sort of double-sided tape will work. Just make sure it is strong enough to keep the pieces together on the router. I used some carpet tape here. Carpet tape is almost like double-sided duct tape. VERY strong and VERY sticky. I wouldn’t use too much or you’ll have trouble getting your pieces apart!

Once the template is attached, it’s time to remove as much material from around the edges of the template as possible. This will make the routing go much smoother and safer! Of course, the key is to get close but not too close to the template. A good 1/16″ is fine.

The bearing on the router will ride along the edge of the template, cutting the piece with precision.

Routing the acrylic

I found that it’s best to use some scrap material when setting the router bit height. Too high and you’ll bite into the template. Too low and you’ll leave a thin piece of acrylic right below the template. The perfect height is where the bottom of the bearing is slightly above the bottom of the template.

The great thing about the template method is once the bit height is set, it is virtually impossible to make bad cuts! The bearing will ride along the edge of the template and cut the material along the shape. Again, the trick here is smooth, consistent pressure and keep the piece moving. You can always back away and approach from a different angle.

Safety is important so never push too hard on the piece and force it into the bit.